Lasers for “Coarse Wavelength Division Multiplexing” (CWDM) applications are expected to operate over a wide temperature range (typically −10 to +85° C.). Changes in bandgap with temperature change the lasing wavelength at approximately 0.1 nm/° C. Each CWDM channel is typically 20 nm wide. This is reduced to approximately 13 nm by filtering; allowing for the operating temperature range and wavelength change over time, a further reduction to approximately 3.5 nm is achieved. Typically, a spread in lasing wavelength of 4-5 nm is seen over a wafer carrying a plurality of such lasers.
The variation in wavelength across the wafer is wider than the specification width, which produces a substantial yield hit (i.e. yield is made appreciably lower). The ability to control or tune the wavelength to offset the effects of temperature will thus significantly improve the yield and thus reduce the laser chip cost.
Integrating heater elements with laser diodes is generally known in the art as witnessed e.g. by U.S. Pat. Nos. 5,173,909 and 5,960,014.
Specifically, U.S. Pat. No. 5,173,909 discloses a wavelength tunable laser diode comprising a temperature variable heater separated from an active layer by a distance less than the thickness of a compound semiconductor substrate. The heater is thus located very close to the active layer, thereby improving the response time of temperature change. This is reported to widen the tunable range of the laser diode.
Any other remarks apart, the arrangement disclosed in U.S. Pat. No. 5,173,909 has the heater element located directly above the active layer of the laser and may therefore be difficult to manufacture.
The arrangement described in U.S. Pat. No. 5,960,014 includes a thin film resistor comprising a bilayer of platinum on titanium. The resistor layer is protected by a layer of dielectric, e.g. silicon dioxide or silicon nitride to reduce degradation from humidity and under high temperature operation. The resistor may be formed on various substrates, including silicon dioxide, silicon nitride and semiconductor substrates. The applications contemplated include integrated resistive heaters for wavelength fine tuning of a semiconductor laser array.
In the arrangement of U.S. Pat. No. 5,960,014, the heating element is positioned on the chip surface at a relatively remote location from the active region. As a consequence, poor heat transfer limits sensitivity and response time.
An object of the present invention is to provide an improved arrangement which may provide a heater element located in the close vicinity of the laser active region, such a result being achieved without making the manufacturing process unnecessarily complex, expensive and possibly less reliable.